The subject matter of this Application concerns the manner of mounting a fan within the extent of a heat sink, while at the same time also providing with that manner a way to mount the entire assembly against a surface from which heat is to be removed. While various styles of heat sinks will fall within the scope of that subject matter, and are therefore of interest, there is a specific one that is of particular interest. We refer to that disclosed in U.S. Pat. No. 5,785,116 entitled FAN ASSISTED HEAT SINK, filed by Wagner on Feb. 1, 1996 and issued on Jul. 28, 1998. That Patent describes a particular type of internal fan heat sink for microprocessors, large power VLSI devices and the like, that dissipate a sufficient amount of power to require a substantial heat sink. The instant invention affords an especially desirable manner of fan attachment and heat sink mounting for that type of internal fan heat sink in particular. However, that heat sink has a number of unique properties that do not readily lend themselves to summary description: it is not itself a garden variety heat sink with a fan grafted onto it. For this reason U.S. Pat. No. 5,785,116 is hereby expressly incorporated herein by reference, so that all the unique properties of that active heat sink, including its manner of operation and manufacture, will be fully available for the understanding of this Disclosure. Such incorporation will be understood to not limit the invention to use with only the heat sink disclosed therein.
Integrated circuits are becoming more and more powerful all the time. Not only is this true in the sense that they do more, and do it faster (e.g., in the field of microprocessors and FPGA""sxe2x80x94Field Programable Gate Arrays), but these newer parts dissipate amounts of power that were unimaginable just a few years ago. For example, there are parts under development that will dissipate one hundred and thirty watts and will need to get rid of the attendant heat through a surface area of about one square inch. There are exotic methods of heat removal that are possible, including heat pipes, chilled water cooling and even actual refrigeration. In the main, these techniques are cumbersome or expensive, and are not suitable for high volume commercial applications in modestly priced retail equipment, such as personal computers and workstations.
The active (meaning fan assisted) heat sink described in the above incorporated Patent to Wagner was developed to deal with this situation. It is a heat sink having a spiral of fins that surround a fan around its circumferential periphery and are in its discharge path. (In other designs the fins are not a spiral, but are straight up and down. We might say they form a ring of straight fins. They occupy the same general region as do the spiral fins, however.) This makes Wagner""s active heat sink a two pass device, since the design draws a portion of its air in through the periphery (one pass) and then discharges it through more fins (second pass). It is a counter flow device, since the path of heat flow is generally opposite to the direction of air flow, so that as air is heated through contact with the fins it encounters still warmer fins as it continues along its path. This ensures greater heat transfer by maintaining a temperature differential between the cooling air and the fins that are to give up their heat to the air. In addition, Wagner""s active heat sink has a number of other desirable properties, such as low noise and an absence of extra mating surfaces that interfere with heat flow.
The preceding several sentences are a brief description of Wagner""s active heat sink, but it is probable that, unless the reader has actually seen one, he or she will not have a completely satisfactory mental image of just what such an active heat sink really looks like. Accordingly, we have included illustrations of Wagner""s active heat sink in the figures, and we have incorporated the Patent to Wagner. However, that still leaves us with the problem of a nice tidy way to refer to it: xe2x80x9cfinned counterflow two pass active heat sinkxe2x80x9d is accurate as far as it goes, but is also pretty cumbersome. Various heat sinks of this design are on the market, offered by Agilent Technologies, Inc. under the trade name xe2x80x9cArctiCoolerxe2x80x9d, but it would be a risky business to rely on that, since we can""t be sure what that term will eventually come to encompass. So, we will do as we have already begun to do above: we shall call the kind of fan-assisted heat sink described above and in the Specification of the incorporated Wagner Patent a xe2x80x9cWagner active heat sinkxe2x80x9d, or depending upon the grammatical needs at the time, xe2x80x9cWagner""s active heat sink.xe2x80x9d By availing ourselves of this coined phrase, we shall avoid much inconvenience. On the other hand, it will be clear that there are other fan assisted heat sinks that may benefit from the invention. When we mean to refer to the entire class of all fan assisted heat sinks that may be so benefitted (Wagner""s included), we shall use the term xe2x80x9cgeneric active heat sinkxe2x80x9d instead. A generic active heat sink need not be a two pass counter flow heat sink. And on the principle that whatever makes for shorter sentences is good, when it is entirely clear that we are referring to is indeed a generic active heat sink, we shall feel free to call it an xe2x80x9cactive heat sink,xe2x80x9d or just a xe2x80x9cheat sink,xe2x80x9d as a further simplification.
It will, of course, be appreciated that as the Wagner active heat sink gains further acceptance and additional needs and applications develop, the exact size, relative shape and so forth will evolve over time. Thus, there are already small ones, medium and large sizes, and extra heavy duty ones, etc. Thus, it will be understood that the specific examples shown in U.S. Pat. No. 5,785,116 (Wagner) are merely illustrative of a general class of Wagner active heat sinks, and such specific details as the number of fins, whether they are straight or spiral, their thickness compared to their height, the number of blades on the fan, whether the thing is tall or squat, etc., are not determined by our meaning of the term xe2x80x9cWagner active heat sink.xe2x80x9d
To continue, then, an active heat sink has a fan and a heat receiver/dissipation element (the xe2x80x9cheat sinkxe2x80x9d part), a particular surface of which must be brought into intimate thermal contact with the source of heat that is to be removed. We thus have the issues of mounting a fan to the heat sink portion, and of attaching the whole assembly to the part to be cooled. In conventional active heat sinks those functions have been performed with separate mechanical structures.
So, for example, and with reference to the view 1 of the prior art Wagner active heat sink shown in FIG. 1, a typical method of assembling a Wagner active heat sink involves gluing a motorized fan 3 into an interior cavity of a (spiral) finned counter flow heat sink 2. Not only does this mean: (A) That one has to discard an inherently valuable chunk of cast or machined metal (2) in the hope of being able to obtain a suitable replacement, when only the motor of the fan has failed (the life in commerce of a suitable class of replacement fan might exceed that of a particular configuration of complete Wagner active heat sink); but also, (B) It increases the time and cost associated with manufacturing the Wagner active heat sink. For example, there are cost and reliability issues surrounding the use of glue: it takes time to set and attention to process variables to ensure proper performance of the adhesive; there is testing on at least some parts in each lot to ensure that the fans are indeed attached (a pull test); and, then another fan operation test to make sure the fan wasn""t damaged during the pull test. Clearly, the use of glue is not as simple as it might seem. It would be desirable if there were quick, inexpensive, reliable and removable way of attaching the motorized fan to an active heat sink.
To continue with the example of FIG. 1, a typical prior art manner of mounting a Wagner active heat sink against a part 6 in need of cooling and carried on a printed circuit board 5, was to machine onto the bottom of the finned portion a downward depending (as would be viewed in the figure, although it is not visible) cylindrical plug having the desired thermal contact area and a suitably smooth butt end. Then that is pressed through a press fit hole in mounting plate 4. The thickness of the mounting plate 4 is less than the length of the cylindrical plug, so that the butt end of the plug is exposed for unobstructed contact. That attaches the spiral finned portion 2 to the plate 4, which in turn has a pattern of mounting holes 9 that are in alignment with corresponding pattern of holes 8 in the printed circuit board 5. A collection of screws 7 (or small bolts, etc.) is then used to compress the assembled Wagner active heat sink 10 against the top of the part 6. It is not so much that this does not work: it does. But it also has some disadvantages. There is the issue of tolerances to be maintained to assure the press fit. There is having to perform the press fit. The mounting plate 4 has a relatively big foot print that takes up space on the printed circuit board 5, to the exclusion useful circuitry that might otherwise be mounted in some of that space, if the foot print were smaller. And finally, there is the issue of what goes wrong if the fasteners are not correctly tightened. The butt end of the cylindrical plug and the top of the part 6 must be in intimate thermal contact. That means that the top surface of the part 6 and the butt end of the cylindrical plug must be parallel as well as touching. Any lack of parallel contact results in extreme loss of thermal conductivity. Springs have been used to assist in allowing the parts to xe2x80x9cover centerxe2x80x9d onto themselves and provide parallel contact over a range of screw travel, so long as certain force limits are not exceeded. But is still has to be done with care, and the springs are extra parts that introduce their own aggravations. They can get lost, or an incorrect spring can be used by mistake, etc. It would be desirable to find a less troublesome way of mounting an active heat sink to the part it is to cool.
While the Wagner active heat sink has been quite successful in the market, techniques that decrease the cost while increasing the utility of a desirable product are always welcome.
It would be especially desirable if there were one low cost and reliable manner of reversible attachment (reversible in the sense that it can be unattached without violence) that accomplished both the mounting of the motorized fan to the heat sink proper, as well as mounting the entire generic active heat sink assembly onto the part to be cooled.
What to do?
A solution to the problem of decreasing the cost and increasing the ease and reliability of removably mounting a motorized fan to either of a generic or a Wagner active heat sink, and then mounting the complete active heat sink to a part to be cooled, is to: (1) Use a motorized fan that engages and registers itself upon and against a top peripheral surface surrounding a cavity where the fan is to be located; (2) Hold the fan in place with one or more resilient mounting clips that span the distance from the top of the motorized fan and an opposing outside bottom surface of the active heat sink; and, (3) Provide the resilient mounting clips with outward projecting mounting tabs that are flexible and resilient and that have mounting holes through which the resilient mounting clips (and the active heat sink they grip) can be attached to an assembly carrying the part to be cooled, such that the bottom of the active heat sink is in good thermal contact with that part. The resilient removable mounting clip or clips may be formed of stainless steel strips suitably punched with holes and bent to shape.